The need for precise positioning of an object is required in many fields of application, including applications in semiconductor manufacturing such as microlithography. As microprocessors become faster and more powerful, an ever increasing number of transistors are required to be positioned on a semiconductor chip. This necessitates closer placement of the transistors and circuits interconnecting them, which in turn requires an ever increasing accuracy in the methods for laying down the circuits on the chip. Thus, there is a need for more precise positioning and maintaining of position, of a substrate during microlithography.
Various systems have been designed to attempt to improve fine positioning and movement control of a work piece. British Patent Specification 1,424,413, assigned to Handotai Kenkyo Shinkokai describes several stage that are supported by flexures and actuated using electromagnetic force actuators. U.S. Pat. No. 3,935,486, invented by Nagashima describes a stage that is controlled using electromagnetic force actuators. In this case, the stage is supported on flexural bearings in 6 degrees of freedom (DOF) and the actuators are used to adjust the position of the stage. Both of these designs utilize flexural bearings to constrain the motion of the stages in 6 DOF. The electromagnetic actuators only provide force; they are not used to control all directions of motion of the stage. Nor is there any disclosure of providing a linear motor driven coarse stage.
Ideally, the bearings for a stage should have infinite stiffness in the directions for which position of the stage is to remain fixed, and zero stiffness along the directions in which the stage is to be moved, to maximum precision and efficiency. Flexural bearings fall far short for the ideal and generally have a stiffness ratio (stiffness in directions to be fixed to stiffness in directions to be moved) of only about 100:1 and possibly up to about 1000:1 but the price of the latter is likely prohibitive in practice. Moreover, a much greater stiffness ratio is desirable.
U.S. Pat. No. 4,952,858 invented by Galburt describes a wafer fine stage that is supported and positioned in 6 DOF by electromagnetic voice coil motors. The motion of the wafer fine stage is entirely constrained using voice coil motors, and this design does not utilize any flexural bearings. Voice coil motors, however, require relatively large amounts of power to generate a given amount of force. The high power requirements of voice coil motors can generate sufficient heat to change the index of refraction of the environment sufficiently to induce error in an interferometer system. Additionally, heat generation can cause expansion of the stage leading to further errors in alignment and control. Further, U.S. Pat. No. 4,952,858 discloses the use of permanent magnets to counterbalance the weight of the fine stage. This counterbalance force is a nonlinear function of stage position, and is thus quite difficult to control accurately.
U.S. Pat. Nos. 5,157,296 and 5,294,854, invented by Trumper describe a wafer fine stage bearing system. This system includes electromagnetic actuators, which act as bearings in 6 DOF. These patents describe control means for the bearings and apparatus for counterbalancing the weight of the stage using either opposed permanent magnets or a heavy oil in which the stage floats. U.S. Pat. Nos. 5,157,296 and 5,294,854 also do not utilize flexural bearings. The electromagnetic actuators in the Trumper patents are arranged in pairs, on opposite sides of the stage, in order to provide stable control. Thus, all forces applied by the electromagnetic pairs are transmitted through the stage, which can result in deformation of the stage.
The counterbalance forces in the Trumper patents may be provided by permanent magnets or by floating the stage in oil. As noted above with regard to the Galburt patent, utilization of permanent magnets results in a nonlinear force curve and corresponding control problems. With regard to floating the stage in oil, oil presents significant problems for a clean room environment typically used for semiconductor processing.
U.S. Pat. No. 5,528,118, invented by Lee, describes a guideless stage for aligning a wafer in a microlithography system, and a reaction frame which isolates both external vibrations as well as vibrations caused by reaction forces from an object stage.
U.S. Pat. No. 5,623,853, invented by Novak, et al., describes a wafer coarse and fine stage for a lithography machine. The coarse stage is a stacked arrangement of linear motor-driven air bearing slides. The fine stage is driven in 3 DOF using voice coil motors. The remaining DOF of the fine stage are constrained using flexural bearings. The use of flexural bearings for the 3 planar DOF limits the servo bandwidth of the stage because the flexural bearings have a limited stiffness in the plane. In addition, the finite stiffness of the flexural bearings out of the plane, distorts the out of plane motion of the stage.
In addition to the above described attempts at providing a superior fine stage design, various attempts have also been made to provide planar motors for use in driving positioners in the field. Disclosures in the field of planar motors include Hinds, U.S. Pat. No. 3 51,196; Hinds, U.S. Pat. No. 4,654,571; Trumper, U.S. Pat. No. 5,196,745; and Chitayat, U.S. Pat. No. 5,334,892. These patents describe planar motors that have significant limitations. For example, the planar motors of Hinds '196 has limited range of motion because each portion of the stationary magnet array can only generate force in a single direction. Thus, each coil array must always be located above the corresponding magnet array. This limits the range of movement for a given size actuator. In addition, the coils and magnets are iron-core and generate sizable attractive forces as well as force ripple. This does not allow for motion in six degrees of freedom because the levitation force cannot overcome the attractive force between the two pieces. Additionally, none of these attempts have combined, or suggested to combine a fine stage operating in conjunction with a planar motor coarse stage.
Hinds '571 suffers from a non-compact design. A large portion of the base of the moving portion of the stage is covered by the air bearing pads and other elements. Only a small portion of the stage is covered with coils. In addition, the coil design is not the most efficient for producing force, since at most only fifty per cent of the coil can generate force. In addition, the moving coil design has a large number of hoses and cables going to the stage, creating a large bias force. Finally, this design does not generate force for a six-degree-of-freedom movement.
Trumper discloses several stage designs with six degrees of freedom. The invention uses conventional coils. Each coil array must be located above a corresponding linear magnet array. This restricts the range of movement for a given sized stage.
Chitayat discloses several planar motor designs, which permit a wide range of motion, but only restricted to translation and rotation in a plane. Thus, the motor of Chitayat is incapable of moving with six degrees of freedom.
Kim and Trumper, in “High-Precision Magnetic Levitation Stage for Photolithography”, American Society for Precision Engineering, 1997 Proceedings, Volume 16, pp. 470-473, discloses the design of a permanent magnet linear motor, for use in a magnetically-levitated wafer stepper stage in which four linear motors provide both suspension and drive forces.
Holmes et al., in “A Long Range Scanning Stage”, American Society for Precision Engineering, 1997 Proceedings, Volume 16, pp. 474-477, discloses a long range scanning stage, having 25 mm×25 mm mobility in the X-Y plane and
Kim and Trumper, in “Precision Control of Planar Magnetic Levitator”, American Society for Precision Engineering, 1998 Proceedings, Volume 18, pp. 606-609, discloses a stage that provides fine motion control in six degrees of freedom. The key element is a linear motor capable of providing suspension and translation forces without contact.
Asakawa, U.S. Pat. No. 4,555,650, discloses a two-dimensional driving device for use in a positioning device for a semiconductor manufacturing apparatus. A magnetic field group is formed by disposing magnetic fields on a plane in a two-dimensional array. At least two coils are distinctly oriented so as to provide a driving force in each of two orthogonal directions. Asakawa, U.S. Pat. No. 4,535,278 also provides a two dimensional driving arrangement for “precision” positioning through use of a planar array of magnetic fields that interact with appropriately oriented coils. Since the devices disclosed in these patents are not capable of positioning in greater than two degrees of freedom, there is a need to provide a finer control in a portioning device.
Sawyer, U.S. Reissued Pat. No. Re. 27,289 discloses a magnetic system for moving a marking tool over a surface for plotting curves. Sawyer, Reissue Pat. No. Re. 27,436 discloses a two-axis magnetic system for driving chart plotters. The Sawyer Reissue patents are not only not directed to the semiconductor-positioning field, but they also are limited to positioning along only two degrees of freedom. Additionally, they use variable reluctance to drive the devices and consequently have cogging forces and relatively low precision.
Trost et al., U.S. Pat. No. 4,506,205 discloses an electromagnetic alignment apparatus for use in aligning wafers in a microlithography system. The apparatus includes three or more spaced magnets that are fixed and interact with three or more spaced coil assemblies that move to effect positioning. As a result, the structure can be moved selectively in three degrees of freedom.
Siddall, U.S. Pat. No. 4,694,477, discloses an apparatus for micro positioning an X-ray lithography mask, in which three piezoelectric transducers are provided for moving a stage plate in the X-Y plane, and three flexure assemblies support the stage plate and move the stage plate along the Z-axis. As noted above, the use of flexural bearings limits the servo bandwidth of the stage because the flexural bearings have a limited stiffness in the plane. In addition, the finite stiffness of the flexural bearings out of the plane, distorts the out of plane motion of the stage.
Reeds, U.S. Pat. No. 4,891,526 discloses a positioning stage for high speed step and repeat alignment of a semiconductor wafer to a mask with six degrees of freedom. Linear bearings are provided on a first plate for movement in the X-direction, and this sub stage is also mounted on an intermediate stage that is mounted on another set of linear bearings for movement in the Y direction. The entire X-Y stage is then mounted on a rotation stage platform.
What is needed that is not provided in the prior art is an improved stage positioner in which higher fine stage mechanical bandwidth is obtained by improving the stiffness characteristics of the bearings/drivers supporting the fine stage. A related advantage to be obtained thereby is the elimination of cross coupling between X or Y axis acceleration forces and Z axis motion. In several embodiments, an advantage is obtained in the ability to control the fine stage through feed-forward actuation of the bearings supporting the fine stage. Another need is to reduce the complexity and improve the performance of a coarse stage on which the fine stage is mounted. A planar motor can provide motion and force in at least two directions, thereby eliminating the need for a set of stacked X-Y stages. This reduces the mechanical complexity and mass of the coarse stage, and increases the stiffness by removing joints between the stacked stages, all of which results in improved dynamic performance of the apparatus.